It was, unfortunately, the reality that, as a Cpl under training on the GST-2 course, you were still a Cpl until you passed out successfully at the end. Then, in my case, you were promoted to Acting Sgt until completion of your TMT-2 course. As a Cpl you were in no way entitled to use the Sgts Mess, & the nearest we got was a courtesy visit during GST-2 to see what we were about to get into, & be informed of the sort of behaviour the 'City Fathers' would expect from newly-minted SNCOs. The few who didn't get through GST-2 simply went back to their units as they were. When I went back as A/Sgt on my TMT-2, we were housed in one of the blocks near the mess, as the mess was deemed not to have enough accommodation for us. We ate in the mess, though.
Compare & contrast with 14 weeks bu**ering about at OCTU, living in an officers mess & being treated as such (albeit with a white band on your hat) - then crashing back into reality as a Cpl.